1. Field of the Invention
The present invention relates to a zoom lens device of a two-lens group structure used in a still camera, a video camera, etc.
2. Description of the Related Art
In a general known zoom lens device of a two-lens group structure, the construction of a zoom optical system is simplified and the zoom optical system is made compact.
A zoom lens of a two-lens group structure has various kinds of constructions. There is a zoom lens of a two-lens group structure in which positive and negative lens groups are sequentially arranged from the side of a photographing field. Such a zoom lens can be generally classified into the following three focusing systems in accordance with a difference in focusing operation.
A first focusing system is a front focusing system. In this front focusing system, a positive lens group is moved to control a focusing operation while a negative lens group is stopped in a position in which the negative lens group is located at a zooming time.
A second focusing system is a rear focusing system. In this rear focusing system, the negative lens group is moved to control a focusing operation while the positive lens group is stopped in a position in which the positive lens group is located at the zooming time.
A third focusing system is an integrally driving system. In this integrally driving system, the positive and negative lens groups are moved to control a focusing operation.
It is important to make a camera compact and increase a zoom ratio so as to improve the portability of a camera and enlarge a photographing condition. It is concretely considered that the camera is made compact and the zoom ratio is increased by reducing a length from a film face within the camera to a front end of a photographing lens.
In the front focusing system and the integrally driving system, the positive lens group is further drawn out at the zooming and telescopic times in the case of a most proximate photographing operation. Accordingly, a length from a film face to the front end of the photographing lens including the length at the focusing time is increased.
In contrast to this, in the rear focusing system, no positive lens group is drawn out. When a distance between a film face and the negative lens group including the distance at the focusing time is equal to zero, the length from the front end of the photographing lens to the film face is set as a minimum limit.
In the front focusing system and the integrally driving system, a drawing-out amount of the positive lens group on a telescopic side is reduced when the lengths from the front end of the photographing lens to the film face in the above three focusing systems are equal to each other. Thus, distances between the positive and negative lens groups are increased in the front focusing system and the integrally driving system. Accordingly, it is impossible to set a focal length longer than that provided in the rear focusing system.
For example, when the camera is concretely designed without considering a macro photograph, a zoom lens capable of performing a zooming operation in the region of a focal length from 38 to 80 mm fulfills an additional function in which the macro photographing operation can be performed in an entire zooming region. In this case, the size of a zooming region on a wide angle side is reduced to secure a moving amount of the negative lens group on a side of the film face so that only a zooming region of the focal length from 40 to 80 mm is set.
As mentioned above, in the rear focusing system, the zooming region on the wide angle side at a normal photographing time is reduced in size to perform a macro photographing operation of low frequency in use. When the lengths from the front end of the photographing lens to the film face in the above three focusing systems are equal to each other, it is impossible to set a shorter focal length on the wide angle side in comparison with the front focusing system and the integrally driving system.